System and method to illuminate and image the inside diameter of a stent

ABSTRACT

An optical system is effective to illuminate and scan an interior wall of an object having an interior bore, such as a stent. The system includes a light source, an object support having a light conducting portion, an image taking lens, and a line scan camera. The interior bore and the light conducting portion of the object support are in axial alignment with a center optical axis of the image taking lens. A drive mechanism engages the object without impacting the axial alignment. Various aspect of this optical system include a rotating wheel or transparent plate as the drive mechanism. The object support may be an opaque rod having a light conducting portion or a transparent rod. Electronics associated with this optical system include a rotary encoder engaging the drive system to drive an electric circuit capable of triggering the line scan camera in response to rotation of the object, thereby building a line by line image of the interior bore and the line scan camera connected to a computer-based imaging system effective to identify cosmetic and functional manufacturing defects.

CROSS REFERENCE TO RELATED APPLICATION(S)

This patent application is a divisional application of U.S. patentapplication Ser. No. 11/714,447 titled “Method to Illuminate and Imagethe Inside Diameter of a Stent” that was filed on Mar. 6, 2007, and isnow U.S. Pat. No. 7,619,763 which in turn claims priority to ProvisionalPatent Application Ser. No. 60/780,763 titled “Method to Illuminate andImage the Inside Diameter of a Stent” that was filed on Mar. 9, 2006.Both U.S. Ser. No. 11/714,447 and 60/780,763 are incorporated byreference in their entireties herein.

U.S. GOVERNMENT RIGHTS

N.A.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to scanning and illuminating systems and methodsfor automated optical inspection. More particularly, to systems andmethods to illuminate and inspect the inner bore of a stent or othertubular or conical device having an interior bore, where the interiorbore may be either a through hole or a blind bore.

2. Description of Related Art

Stents are small wire mesh tubes used to hold open compromised arteriesand other fluid conduits within a human body. The critical use of thesedevices and their small size requires stents to be manufactured to thehighest possible quality standards.

Balloon expandable stents are vulnerable to failure during deployment. Asharp edge may puncture the balloon under high pressure causing adeployment failure. Also, arteries and vessels in the body flex and bendand a deployed stent must conform. Manufacturing defects may cause apoint of weakness in the stent that is vulnerable to fatigue and failureas this point of weakness is repetitively bent or flexed in the vessel.Still further, if a portion of the stent breaks off and travels throughthe bloodstream, the patient is a risk of a stroke if this broken piecetravels to the brain and lodges in an artery.

In view of the potential for catastrophic failure, rigorous inspectionof the stent is required prior to deployment. Within the industry,visual inspection of stents has historically been done by humanoperators utilizing a microscope at 40× to 80× magnification. The stentsare typically placed between two rollers and rotated under themicroscopes while the human operator observes. An automated inspectionsystem for measuring the dimensions of a stent and inspecting exteriorsurfaces is disclosed in U.S. Pat. No. 6,606,403, “Repetitive InspectionSystem with Intelligent Tools,” that is incorporated by reference in itsentirety herein.

Summarizing the current state of the art and its deficiencies, toachieve an automated method for inspection, a complete image of thestent needs to be quickly taken in good focus with clear contrast. Thecurrent state of the art is to use a stereomicroscope such as theOlympus SZ40 and ringlight. The stent is placed between two rollers andmanually rotated. Illumination is provided from above with a fiber opticringlight. While a video camera can be used with such a microscope, asthe stent rotates between the rollers it does not always turn smoothlyso it can move in and out of focus. Human operators often readjust thefocus knob of the microscope to accommodate this, but this would bedifficult and time consuming for an automatic method. As such, systemsand methods need to be developed to rotate the stent in a well-definedand fixed geometry so as to always keep the stent in focus.

Further video cameras that can be used in conjunction with microscopesgenerally can only bring a very small section of the stent in focus atone time. To image every section of the stent with such an approachwould require a very large number of individual images. This would beimpractically time consuming to generate. Thus a method is required togenerate a complete image of the inside diameter of the stent in just afew seconds. The current state of the art also uses a ringlight thatilluminates the top outer diameter of the stent as well as the innerdiameter thus creating a confusing image with excess glare.

There remains a need for an automated system and method to illuminateand image the inner bore of a stent and thereby facilitate theinspection of that inner bore.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the invention is directed to an optical system that iseffective to illuminate and scan an interior wall of an object having aninterior bore. This system includes a light source, an object supporthaving a light conducting portion, an image taking lens, and a line scancamera. The interior bore and the light conducting portion of the objectsupport are in axial alignment with a center optical axis of the imagetaking lens. A drive mechanism engages the object without impacting theaxial alignment.

Various aspect of this optical system include a rotating wheel ortransparent plate as the drive mechanism. The object support may be anopaque rod having a light conducting portion or a transparent rod.Electronics associated with this optical system include a rotary encoderengaging the drive system to drive an electric circuit capable oftriggering the line scan camera in response to rotation of the object,thereby building a line by line image of the interior bore and the linescan camera connected to a computer-based imaging system effective toidentify cosmetic and functional manufacturing defects.

One use for the optical system is to illuminate and inspect stents.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates illumination of an inner sidewall of astent in accordance with an embodiment of the invention.

FIG. 2 shows a beamsplitter mounted inside a slotted rod to provide morecomplete illumination of the inner sidewall of the stent.

FIG. 3 shows an alternative embodiment utilizing a flat stage to drivethe stent around slotted rod.

FIG. 4 shows another alternative utilizing two pairs of drive rollersthat move a transparent stage while capturing and rotating the stent.

FIG. 5 shows another embodiment that utilizes three drive rollers tocapture and rotate the stent.

FIG. 6 shows another embodiment for rotating a stent.

DESCRIPTION OF THE INVENTION

One embodiment of the invention utilizes a linear array camera, largerarea taking lens with relatively shallow depth of field, and a drivedevice to rotate a stent in a fixed location in space so as to keep theinner diameter clearly in focus across the field of view. This takinglens preferably is effective to image a substantial portion of the stentalong its axial length, typically 10 mm or more. An encoder is affixedto the rotary drive device and is used to trigger the camera. Anillumination source is geometrically configured to avoid putting lighton an upper outer diameter of the part. The numerical aperture of thetaking lens is at least as large as that of current manual microscopes(NA=0.1 or higher) for the purpose of bringing the outer diameter out offocus while the inner diameter is in sharp focus. The result of thisoptical configuration is a flat unrolled image of the stent innerdiameter. This image is then analyzed for quality using grayscale imageprocessing techniques available on an image processing board such as theOdyssey from Matrox Imaging, Montreal, Canada.

One aspect of this embodiment is that the drive device effectivelyrotates the stent while providing an unobstructed view of the stentinner diameter. A number of different methods for this are described.One method is to mount the stent on a clear rod or tube and use acorrecting cylindrical optical element to reverse any optical distortioncaused by the rod or tube. Another method is to mount the stent on aslotted metal rod and propel it around this rod by a compliant roller orslide mechanism. The camera can then image the inner diameter of thestent by viewing through this slot. To improve the image, if the drivemechanism is partially transparent or translucent, illumination can besent up through the drive mechanism and light reflected off a portion ofthe slotted rod. This approach achieves the desired goal of illuminatingonly the inner diameter of the stent and not the outer diameter.

Another embodiment is to capture the stent firmly against two rollerswith a third roller or with a clear plate that moves synchronously withthe rotating stent. Two of the rollers could be used as reflectors todirect light to the ID of the stent without hitting the outer diametersurface.

FIG. 1 illustrates a first system that illuminates an inner diameter(ID) of a stent 1 while avoiding illumination of the outer diameter (OD)of the stent. Illumination on the OD creates substantial stray lightdegrading image quality and contrast. The stent 1 is mounted on a rigidslotted rod 5. A typical cardiovascular stent 1 has an inside diameterof about 1.5 millimeters and the slotted rod 5 has an outer diameterslightly less than that so the stent 1 does not collapse, but rotationof the stent 1 does not cause rotation of the slotted rod 5. Nominally,the outside diameter of the slotted rod 5 is 0.1 millimeter less thanthe inside diameter of the stent 1. A lens 6 has a depth of focuseffective to image an interior wall 2 of the stent 1 onto a line scancamera 8 without also bringing the higher OD portion of the stent intofocus. A drive device, such as rotatable drive wheel 12, contacts thestent 1 and is effective to rotate the stent around the slotted rod 5 ata desired rate. Light from a light source 15 passes through atranslucent section 16 of drive wheel 12. An opaque projection 18 blocksexcess light from the light source 15 that would otherwise provide toomuch contrast to line camera 8.

The slotted rod 5 is preferably formed from a rigid, opaque, materialsuch as metal. The slotted rod is supported a fixed distance from theline scan camera 8 and the lens 6. The fixed in place slotted rod 5 hasa slot 7 axially extending a length approximately equal to the viewingfield of the lens, nominally 15 mm. The slot 7 enables line scan camera8 to view through the slot 7 and take an image of the ID of the stent 1.While the slot may be cylindrical or any desired shape, an hourglassshape, or other shape effective to provide inwardly directed sidewallseffective to receive and reflect light from the light source, ispreferred. The hourglass shape is formed by the slot having a maximumdiameter at the surfaces of the rod and then a taper to a minimum at apoint within the slotted rod. As shown in FIG. 1, the maximum diametersneed not be the same on either side of the rod, nor need the rate oftaper be similar such that the minimum diameter point is not necessarilyat the center of the rod. Preferably, the minimum diameter point of theslotted rod 5 is located substantially closer to the bottom of the rodthan the top so that this bottom portion functions like a reflector forlight passing through the rotatable drive wheel 12 from source 15.

While the preferred slotted rod is formed from a rigid, opaque,material, fabricating the slotted rod from a clear or translucentmaterial such as ceramic or quartz could be done as well. If light isdirected from any angle, such a rod would itself become a glowing sourceof light. While this would accomplish the objective of providing a fixedarbor that provides illumination only on the inner diameter of a stentand not the outer diameter and keeps the ID being imaged at a preciseposition with respect to the camera, it would likely be a more difficultrod to produce and likely less durable in a manufacturing environment.

To rotate the stent 1 around the fixed rod 5, a motor driven wheel 12that is preferably covered with a compliant, typically rubber, coating14 is contacted with the stent 1. Preferably, the drive wheel 12contacts with the stent at a point in alignment with slot 7. Thisprovides a highly registered location for the stent image to be taken ata fixed distance from the lens 6 so as to accommodate the shallow depthof focus of a high numerical aperture (NA) lens 6. This numericalaperture is typically greater than 0.1. The driving wheel 12 includes anencoder that communicates with a motion controller that, for example,processes the stored values of a) the diameter of the drive wheel, b)the diameter of the stent and c) the resolution of the encoder tocalculate the appropriate times to trigger the line camera to take aline and provide substantially square pixels.

It is preferable for the drive wheel 12 to have a substantially largerdiameter than the stent 1, nominally by at least a factor of three toone. A section 16 of the drive wheel is substantially translucent toenable light to be transmitted through the drive wheel 12 and impinge onthe slot 7 of the fixed slotted rod 5. By having the diameter of thedrive wheel 12 substantially larger than the diameter of the stent, thestent may be rotated through 360° while contacting the substantiallytranslucent section 16 of the drive wheel circumference. The insidesurface of the slot 7 adjacent translucent section 16 has a concaveshape along its axial length. This concave shape collects light anddirects it onto the interior wall 2 of the stent 1. This approach avoidsshining light on top outer diameter 3 of the stent. If illumination wasdelivered to the stent 1 from the opposing side adjacent camera 8through the slot 7, there would be excessive stray light reflected intothe lens from the top outer diameter 3 of the stent 1.

A line scan camera 8, such as the P2 6k manufactured by DalsaCorporation of Waterloo, Ontario, Canada, is deployed to build up aline-by-line image of the interior sidewall 2 of the stent 1. To imagethe interior wall 2, a lens 6 of sufficiently high numerical aperture isemployed to bring the interior sidewall of the stent 1 in clear focus,while leaving the nearer to lens 6 outer diameter 3 of the stent out offocus. The higher the numerical aperture, the shallower the depth offocus, therefore the preferred embodiment of this invention must holdthe stent rigidly enough to keep the stent in good focus despite thegenerally shallow depth of focus of such a lens.

Exemplary of the numerical aperture and the resultant mechanicalprecision with which the stent 1 should be rotated in the field of viewof the line scan camera 8, consider a typical cardiovascular stent witha 1.5 mm diameter. Assume that a camera and lens are looking down on thestent as it lies flat on a surface. Then for the top of the stent, theouter diameter, to be sufficiently out of focus so as to not in any waydistort the image of the lower inner diameter the depth of focus mustbe, on the order of 5% of this 1.5 mm diameter or 0.075 mm. The actualgeometry of the given stent does influence on this percentage. Thedenser the stent, the smaller the depth of field must be avoid avingetting effect from the upper outer diameter, but 5% by length of thediameter seems a reasonable value for most stents. If we use theconventional formulas for depth of focus:Depth of Focus=1/NA²,   1then for a depth of focus of 75 microns=1/NA², NA=0.11. For a more densepart, a depth of focus closer to 25 microns is required. This yields alens NA of 0.2. So then an aspect of this system is to mechanicallyrotate the stent under a line scan camera and precisely register themoving stent under the camera and lens to within 25 to 75 microns. Oneeffective system for mechanical rotation is an ADRS-100 rotational stagemanufactured by Aerotech Inc. of Pittsburgh, Pa. The rotating drivewheel 12 can be affixed to a mechanical stage and aligned to run truewithin the focus of the lens. The slotted rod 5 can be rigidly mountedto an assembly holding the lens 6 and camera 8 so that the slotted rod 5can be properly aligned to the camera 8 and lens 6 and those three itemscan be constrained to maintain the ID of the stent in focus at alltimes. Also these three items can be all moved away from the drive wheel12 as one mechanical package to allow loading and unloading of theslotted rod.

A further enhancement to the system is an opaque projection 18, such asa thin metal rod, disposed between the light source 15 and the drivewheel 12 substantially parallel to the fixed slotted rod 7. The opaqueprojection 18 blocks direct rays from the light source 15 that wouldotherwise travel unhindered towards the line scan camera 8. Theseunhindered rays would produce too much contrast between areas ofmaterial on the stent inner sidewalls and open areas. Such high contrastwould cause camera blooming. An alternative method to avoiding excessivecontrast is to split the light source into two separate elements eachplaced slightly off the main optical axis and aimed at the reflectiveareas at the bottom of the slotted tube.

FIG. 2 shows a beamsplitter 4 mounted inside slotted rod 5 to providemore complete illumination to the interior wall 2 of stent 1. Thebeamsplitter 4, exemplary is a partially reflective mirror, within theslot 7 reflects some rays of the light transmitted through the drivewheel 12 onto the stent 1 and provides a more complete and uniformillumination. A slightly more complicated embodiment of thisbeamsplitter approach is to place the beamsplitter 4 in the slot 7 andadd a second slot 9 in the rod 5 at right angles to the first slotsolely for the purpose of delivering light to the beamsplitter 4 fromthe side.

FIG. 3 shows an alternative embodiment where a flat stage, such as aclear plate, 11 is used to drive stent 1 around slotted rod 5. The flatstage 11 is capable of linear motion and replaces the rotating drivewheel. An advantage of this approach is that the calculation of thecamera trigger pulse rate is simplified. A second advantage is that aflat glass element can be used and coated with a thin layer ofcompliant, translucent material to be the driver for stent rotation. Oneminor disadvantage may be the presence of a large mechanical object inclose proximity to the rigid rod could complicate the loading andunloading of the stent for the operator.

Other means to rotate the stent under the high NA lens with the abilityto hold the stent at the appropriate distance from the lens withsufficient precision can be suggested by those skilled in the art. Welist here some of those approaches.

FIG. 6 shows one method is to first place the stent on an opticallyclear cylindrical tube or within an optically clear tube 20. Acylindrical lens element 21 can be then used as part of the taking lensto compensate for any optical distortion caused by the rod or the tube.The clear rod or clear tube can then be rotated by a motor underneaththe camera and lens and an image created. Light can then be directedfrom either side of the clear tube or rod at the ID of the stent bysources 15, thus avoiding the top of the OD of the stent and theinherent stray light caused by so illuminating this section of thestent.

A commonly used drive mechanism to rotate a stent under a manualmicroscope is a pair of counter rotating rollers. Typically theserollers are at least three times the diameter of the stent so that thestent tends to nest between the rollers. After processing and manualhandling, stents can sometimes take on a form that is not perfectlycylindrical. In such a case the interior sidewall of the stent would notrotate with the desired degree of precision required by the high NA lensdescribed above. As shown in FIG. 4, to achieve this degree of placementprecision a first pair of drive rollers 23 rotate the stent 1 under theview of line scan camera 8. A second pair of drive rollers 25 movetransparent flat stage 11 synchronously and capture and rotate stent 1.The transparent flat stage, such as a moving glass cover slip, issupported by the second pair of drive rollers 25 that move it insynchronous motion with the rollers turning the stent. The second pairof drive rollers 25 is at an effective height with respect to the firstpair of drive rollers 23 so that a slight compression is placed on thestent 1. This compression will keep the rotating stent well registeredwith respect to the depth of focus of the lens 6.

An alternative roller approach is illustrated in FIG. 5. Three rollers33 capture the stent 1 with each roller 33 exerting a slight pressure onthe stent keeping it registered under the lens 6 to the properprecision. One or more light sources 15 reflect light off surfaces ofthe rollers 33 to illuminate the inner sidewall 2 of the stent. Thegeometry of the rollers 33 and the light sources 15 is such that none ofthe light from these sources impinges on the top OD surface of the part.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

We claim:
 1. An optical system effective to illuminate and scan aninterior wall of a mesh object having an interior bore, comprising: alight source effective to illuminate said interior wall; a rigid supportrod having a diameter effective to prevent collapse of said mesh object;wherein said support rod has a cross section having an optically clearviewing section that is flanked on a first and a second side by anon-transparent section; a source of linear motion, having a compliantsurface to transfer motion to said mesh object, that rotates said meshobject around said support rod; an image taking lens disposed externallyto said mesh object and configured to view said interior wall throughsaid optically clear viewing section of said support rod and having anumerical aperture greater than 0.1 so that when said interior wall isin focus, an exterior wall on an opposing side of said mesh object isout of focus; and a camera optically aligned with said image taking lensand effective to scan said interior wall.
 2. The optical system of claim1 wherein said light source is out of optical alignment with said cameraand said between said mesh object and said image taking lens.
 3. Theoptical system of claim 1 wherein said support rod includes a beamsplitter effective to reflect light from said light source onto saidinterior wall.
 4. The optical system of claim 1 wherein said support rodand said source of linear motion are effective to hold said mesh objectwith sufficient precision to maintain said interior wall in focus. 5.The optical system of claim 4 wherein said source of linear motion is aflat plate.
 6. The optical system of claim 5 wherein said flat plate isclear.
 7. The optical system of claim 5 wherein said flat plate istranslucent.
 8. The optical system of claim 1 wherein said mesh objectis a stent.
 9. The optical system of claim 1 wherein said source oflinear motion is coupled to an encoder driving an electronics circuitcapable of triggering said camera in response to rotation of said meshobject about said support rod.
 10. The optical system of claim 1 whereinsaid camera is a line scan camera effective to build a line by lineimage of said interior bore.
 11. The optical system of claim 1 whereinsaid taking lens has an optical design that compensates for any opticaldistortion caused by the support rod.